Superplastic forming (SPF) is a known process that involves heating and then forming workpieces through the use of dies. SPF relies on superplasticity, a material property that allows certain metals and alloys to be plastically deformed without rupture well beyond their normal limits within specific temperature ranges and strain rates.
With SPF processes, highly complex and contoured monolithic parts can be formed at a relatively low cost from such materials as titanium, steel and aluminum by reducing total part count and the assembly of details. Common applications of SPF include the manufacturing of parts for aircraft, missiles and space vehicles.
To heat the workpiece in a forming process, like SPF, hot forming, thermoforming, consolidation, or heat treatment, induction heating systems have been developed. For example, U.S. Pat. No. 5,410,132 entitled “Superplastic Forming Using Induction Heating” discloses apparatus and methods for inductively heating and superplastic forming a workpiece. U.S. Pat. No. 5,683,608 entitled “Ceramic Die for Induction Heating Work Cells” discloses a ceramic die for use in an induction heating workcell that incorporates segments of the induction coil in a spaced array within a cast ceramic or phenolic body. A peripheral compression frame, typically of phenolic, surrounds the die body and applies a compressive load to the die body through lateral and transverse reinforcing rods that are cast into the die body. Dies close to direct heat in a workpiece that is located at about the center of the induction coil. The contents of U.S. Pat. Nos. 5,410,132 and 5,683,608 are each incorporated herein by reference in their entirety as if fully set forth herein.
U.S. Pat. No. 5,728,309 entitled “Method for Achieving Thermal Uniformity in Induction Processing of Organic Matrix Composites or Metals” and U.S. Pat. No. 5,645,744 entitled “Retort for Achieving Thermal Uniformity in Induction Processing of Organic Matrix Composites or Metals”, disclose methods for induction heating forming a workpiece with a ceramic die. The contents of U.S. Pat. Nos. 5,645,744 and 5,728,309 are each incorporated herein by reference in their entirety as if fully set forth herein. In each of U.S. Pat. Nos. 5,645,744 and 5,728,309, susceptors that enclose the workpiece in a heating zone are used and have a Curie Temperature that is equal to or substantially the same as the desired forming temperature for the workpiece. Temperature uniformity of the workpiece is readily achieved because the magnetic permeability of the susceptor falls to unity (i.e., the susceptor becomes paramagnetic) at the Curie Temperature, causing the temperature of the susceptors and the workpiece to be maintained at the Curie temperature. Accordingly, thermal uniformity of the heated workpiece during the forming process can be achieved irrespective of the input power fed to the induction coil by judiciously selecting the material for the susceptor. The workpiece cannot overheat if energy is used efficiently. This, in turn, allows for improved control and improved temperature uniformity in the workpiece resulting in the production of better products during the forming process.
While the advancements of induction heating systems described above have done much to reduce fabrication costs, cycle times, part and fastener counts as well as improving energy efficiency and the quality of the finished articles, among other advantages, the inventors have recognized that several issues remain. For example, the typical candidate part complexity requires a complexly shaped susceptor on the part side of the die. However, complexly shaped susceptors can be rather difficult and costly to produce. Accordingly, the inventors have recognized a need for devices and methods that eliminate the need for a complexly shaped susceptor in an induction heating systems.
In addition, the inventors have also recognized that the sealing welds presently used in induction heating systems to seal a pressurizing cavity adjacent the workpiece increase the time needed for preparation and introduce trimming (i.e., cutting of the workpiece and/or sealing welds) as a necessary step to recover the completed part. Accordingly, the inventors have recognized a need for even more efficient ways of sealing a pressurizing cavity adjacent the workpiece in induction heating systems.